1. Field of the Invention
The invention relates to a night vision device worn on the user's head. More precisely, the invention is a device making use of a standard light intensifier and used in a compact night vision telescope.
2. Description of the Prior Art
The pupil of the eye collects the light rays emitted by the landscape to form an image on the retina. At night, the emitted light intensity is too low for the naked eye to achieve good perception of the environment, yet it remains sufficient to enable an image intensifier to form a visible image of the landscape electronically.
In a light intensifier, or image intensifier tube, the electrons emitted by a photocathode receiving photons from the landscape are multiplied, accelerated and directed onto a screen where an intensified image of the landscape, is formed. In a night vision device, the intensifier is associated with an objective that focuses the scene observed on the photocathode and with an eyepiece that transports an intensified, collimated image of the scene to the user's eye.
Today, the most common night vision devices are night vision binoculars in which the objective, the light intensifier and the eyepiece are aligned in front of the user's eye. The objective forms an image of the scene on the photocathode but it simultaneously imposes a rotation of 180° (the landscape appears upside down on the photocathode). To present the user with a visible image the right way up, a night vision binocular also includes an optical system to rotate the intensified image through 180°, generally a twisted bundle of optical fibers known as an inverter fibers bundle. Standard light intensifiers are widely commercialized; they incorporate a bundle of inverter fibers.
Night vision binoculars can be mounted on a helmet, notably that of an aircraft or helicopter pilot. However, these devices mounted in front of the user's eyes are inconveniently large. Their weight and the position of their center of gravity seriously increases the risk of injury to the pilot's head, notably in the event of sudden, unprepared ejection from the aircraft.
To reduce the overall size of the object mounted in front of the user's eyes, night vision binoculars incorporating optical deflections have been proposed. The U.S. Pat. No. 4,653,879 describes binoculars with an optical circuit positioned in a plane perpendicular to the viewing direction and in which the light is intensified; a mixer superimposes the intensified image on the user's direct view. The complete intensified optical channel includes 6 deflecting mirrors or prisms. Such binoculars a still heavy and too bulky to be placed inside the user's helmet.
Other night vision binoculars have been proposed with 4 optical deflections; each intensified channel includes an objective that performs two deflections, a specific light intensifier without inverter fibers that is positioned vertically and to one side of the user's eye, and an eyepiece that performs two further deflections. These binoculars are compact, light and compatible with the safety constraints imposed by the need for possible ejection of the wearer. However the use of a specific light intensifier results in high costs, in particular those of the certification procedure associated with the use of such binoculars in certain aircraft. The standard intensifier, although it includes an additional bundle of optical fibers compared with the specific intensifier, generally proves to be a much less expensive choice. The replacement of the specific intensifier by a standard intensifier in such binoculars would cause the intensified image of the landscape to be inverted; the addition of a block of inverter fibers to correct this inversion would excessively increase the overall height of these binoculars by raising the objective; binoculars including such a block are not compact.